Essentially, mustard (or "prepared mustard" as it's sometimes called in recipes) is a combination of mustard seeds, either whole or ground, and some sort of acidic liquid like wine or vinegar. You can make a very basic yellow mustard with nothing but mustard powder (which is just ground mustard seeds), vinegar, a little bit of water and some salt. The only thing to remember is that you really need to let your mustard sit overnight before you use it or the flavors will be off.
Also, if you're using whole mustard seeds, you will need to soak them for about 48 hours.
When making mustard, you'll probably use some combination of yellow mustard seeds and brown mustard seeds, as well as mustard powder. Whole seeds give your mustard a crunchier texture while mustard powder will produce a smooth mustard. Liquids can include anything from wine vinegar to wine to beer or even cognac.
With whole mustard seeds, you can grind them up in a spice grinder first, or you can soak them and mix up the mustard in a blender which will purée the mustard to whatever degree you like.
Other ingredients that you can try in your homemade mustard include honey, brown sugar, ground allspice, whole caraway seeds, Worcestershire sauce and Tabasco. You can also add fresh chopped herbs, chopped shallots, minced garlic — as you can see, you're only limited by your imagination. The main thing to remember is that you need to soak the mustard seeds for two days, and you need an acidic liquid to stabilize the flavor and pungency of the mustard seeds.
Here's a simple recipe for mustard that you can experiment with. Also see: Join Me on Facebook | Follow Me on Twitter